by B. K. Ellis, J. A. Craft and J. A. Stanford, updated 12/2012
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A Note From the Authors:
Maintaining clean, clear water is a quality of life issue for Montana. We cannot know conditions without accurate measurements taken routinely year after year. Government support for monitoring has been drastically reduced during the last decade. A fund to help continue the necessary level of sampling to assess trends in water quality for Flathead waters was established by the FLBS, the Flathead Protection Association and others a few years ago and in 2011, an anonymous local donor agreed to match dollar for dollar all funds raised up to $1 million over a period of three years. The match will be endowed thus supplementing limited government funding and providing long-term support for water quality monitoring of Flathead Lake and many basin tributaries. Continuing these monitoring efforts is essential to ensure that the Flathead Lake of the future is as spectacular as the one that we enjoy today.
For more information, visit our Flathead Lake Monitoring Challenge Grant page.
The Biological Station has carefully documented the status of water quality in Flathead Lake and its tributaries since the Station was founded in 1899. In the early days, studies were periodic. Since 1977, measures have been obtained about monthly by the Biological Station using standardized protocols. These studies have been the technical background for development of a Total Maximum Daily Load (TMDL) allocation for the purpose of managing nutrient loads reaching Flathead Lake.
Based on Station research, the Flathead Basin Commission (FBC) recommended the following interim targets for the protection of water quality in Flathead Lake:
- 1) no increase in the biomass of lakeshore periphyton,
- 2) no measurable blooms of Anabaena flos-aquae (or other pollution algae),
- 3) no declining trend in oxygen concentrations in the hypolimnion, and
- 4) average annual concentrations of the following variables in the photic zone of the Midlake Deep site in Flathead Lake will not exceed the values indicated:
- * chlorophyll a - 1.0 ug/L (1 microgram per liter)
- * primary production - 80 g C m-2 yr-1 (80 grams of carbon per square meter per year)
Fig.1 - Mean periphyton biomass as chlorophyll a (ug cm-2) + 1 standard deviation at 5 m depth in August of each year at the two long-term monitoring sites on Flathead Lake.
In the 2011 water year (WY; October 1, 2010-September 31, 2011), periphyton biomass (algae growing on rock surfaces at a depth of 5 m) was within the range of values reported since monitoring began in 1999 (Figure 1). Mean periphyton biomass at the "B" Beach site was the lowest ever recorded for that site and was very similar to the mean for the Horseshoe site. There was no significant trend in periphyton biomass at "B" Beach for the period of record (1999-2011) but a statistically significant increasing trend at the Horseshoe site.
No visual evidence of an algal bloom was detected in the summer and fall of 2010 or 2011, but qualitative assessment will have to be confirmed after enumeration of surface phytoplankton samples. Additional funding is warranted to examine possible factors that cause the toxic blue-green Anabaena to flourish in certain years, to gain insight into the conditions that favor the growth of this noxious species.
Dissolved oxygen concentrations in the bottom waters at the Midlake Deep site have improved. The declining trend in percent saturation of dissolved oxygen reported in recent years is no longer statistically significant. In WY 2010 and 2011, instrumentation equipped with a dissolved oxygen sensor was deployed 30 times from the Jessie B research vessel. Those manual measures indicated that percent oxygen saturation approximately 1 m from the bottom at the long-term Midlake Deep monitoring site dropped to a low of 80% and 86% in 2010 and 2011, respectively. However, since late 2011 the Flathead Lake Biological Station has been measuring dissolved oxygen concentrations a few meters from the bottom 4-6 times a day. Two new instrumented buoys now relay data (via satellite) near real-time thus improving our ability to detect periods of low oxygen throughout the late summer and fall. The buoys are located at the long-term Midlake Deep monitoring site west of Yellow Bay and at another site in the deep trench west of Woods Bay called Midlake North. The oxygen sensor on the automated profiler indicated that dissolved oxygen saturation a few meters off the bottom dropped to 77.5% at the Midlake Deep site in 2012. You may view current weather and underwater profiler data from the FLBS Weather page.
Fig. 2 - Long-term annual mean (thick bar) and range of annual means (thin bars) for nutrient and chlorophyll a concentrations of 0-30m integrated samples collected from 1988 to 2010 at the Midlake Deep site on Flathead Lake. Integrated means were calculated for each water year (i.e., October 1- September 30). Integrated mean concentrations for the 2011 water year, October 1, 2010 to September 30, 2011, (circles) are also presented for comparison.
Fig. 3 - Mean annual pelagic primary productivity (g C m-2 yr-1) at the Midlake Deep site for Flathead Lake from 1978 to 2011. Bars represent minimum and maximum yearly estimates.
The WY 2011 annual mean total nitrogen concentration was above the long-term average for the Midlake Deep site while total phosphorus was below the long-term average (Figure 2). The mean midlake concentration of total nitrogen (125 ug L-1) was identical to that measured for WY 2010 and values for those two years represent the highest annual means recorded for integrated 0-30 m water samples since integrated sampling of the photic zone began in WY 1988. The mean nitrate nitrogen concentration was slightly above the long-term average. Water year 2011 means for the other analytes (i.e., chlorophyll a, soluble reactive phosphorus, ammonium nitrogen) were quite similar to the long-term (1988-2010) annual means.
In WY 2011, the average annual chlorophyll a concentration did not exceed the TMDL target of 1 ug L-1 (see Figure 2). The annual rate of primary production at the Midlake Deep monitoring site in WY 2011 was 95 g C m-2 yr-1 (Figure 3), a value that was somewhat lower than the post-Mysis mean of 99 g C m-2 yr-1, but exceeded the TMDL target by 19%. With the exception of water years 1994 and 2008, annual primary productivity in Flathead Lake has been at least 10% greater than the FBC target since 1989, and in 1998 exceeded the target by 55%. This target variable requires understanding of food web dynamics and cannot be interpreted independent of those dynamics. Funding is actively being sought for refinement of a food web model for Flathead Lake aimed at understanding the dynamics of foodweb interactions and the linkages of increasing nitrogen in the catchment and lake response variables, such as primary productivity.
Dramatic alteration of the composition of at least 3 trophic levels (e.g., fish, zooplankton and algae) of the lake food web occurred during the establishment of Mysis diluviana in the mid to late 1980s (Ellis et al. 2011). This essentially resulted in a lake with a different biological community which has likely altered nutrient cycling. Given the influence of the changing food web on lake response variables (e.g., primary productivity, chlorophyll a) we recommend that TMDL targets be revised to reflect trends for the post-Mysis period only. There are also many indirect effects that are the consequence of the establishment of Mysis in Flathead Lake and those interactions are complex. Additional funding is being actively pursued to model the changes that have occurred in the food web and its effect on the TMDL target parameters.
Reference: Ellis, B. K., J. A. Stanford, D. Goodman, C. P. Stafford, D. L. Gustafson, D. A. Beauchamp, D. W. Chess, J. A. Craft, M. A. Deleray and B. S. Hansen. 2011. Long-term effects of a trophic cascade in a large lake ecosystem. Proceedings of the National Academy of Sciences USA 108(3):1070-1075.